Microwave linear accelerators which use oscillating electric fields to accelerate charged particles (such as electrons) have been used for years as a way to overcome the maximum voltage limitations of static accelerator fields. In a microwave linear accelerator, a stream of electrons is typically passed through a set of microwave cavities containing oscillating electric fields. These oscillating electric fields accelerate the electron stream. Because the accelerating electric fields in these cavities are oscillating periodically, they are only in the correct direction for half the microwave period. To ensure that the fields accelerate rather than decelerate the electron stream, the cavities containing these fields are made short enough so that an electron can completely traverse the length of the cavity before the cavity field reverses to the unwanted direction.
Such known microwave linear accelerators have certain problems. One significant problem is that the short microwave cavity length limits the acceleration force that can be applied to the electrons. This problem has been dealt with in the past by providing additional cavities phased such that the accelerated electrons will find the electric field in the correct direction during the electrons' transit through each successive cavity. This solution increases the amount of acceleration force, but also increases the size and complexity of the linear accelerator.
Another problem with such known microwave linear accelerators relates to their efficiency. In a short linear accelerator, the electron source (e.g., an electron gun) typically produces a continuous stream of electrons. However, only a fraction of these electrons that happen to be properly timed will be successfully accelerated by the linear accelerator. Electrons not properly timed will not be correctly accelerated, and will eventually hit the cavity walls. Thus, discrete bunches and/or batches of successfully accelerated particles will emerge from the linear accelerator at every microwave cycle as opposed to a continuous stream of accelerated electrons. This effect translates into lower accelerated beam power.
Another type of accelerator is known as a "cyclotron accelerator." When people hear the term "cyclotron" they often think of huge systems spanning several miles used to generate extremely high energy particles for "smashing atoms." However, not all cyclotron accelerators are huge. Generally, a "cyclotron" is a circular particle accelerator in which charged subatomic particles generated at a central source are accelerated spirally outward in a plane perpendicular to a fixed magnetic field by an alternating electric field.
Some past known cyclotron accelerators utilize transverse-electric (TE) electromagnetic modes to produce acceleration of an electron beam immersed in an axial focusing magnetic field. Such cyclotron wave accelerators accelerate a charged particle in the direction of power flow of the electromagnetic wave energy in a manner such that the frequency of the wave as seen by the particle is Doppler shifted to a lower value. The decrease in frequency as seen by the particle is exactly the amount necessary to compensate for the lower cyclotron frequency that results from the relativistic increase in particle mass. To operate efficiently, such past cyclotron accelerators require that the following condition is met, EQU .OMEGA..sub.c &lt;.omega. (1)
where .OMEGA..sub.c is the relativistic cyclotron frequency and .omega. is the angular frequency of the wave. PA1 a rotating-wave accelerator that provides a continuous stream of monochromatic charged particles employing a relatively short (e.g., TM.sub.110) rotating mode cavity with a suitably up-tapered axial focusing magnetic field. PA1 a rotating-wave accelerator using a transverse-magnetic rotating wave mode, TM.sub.110, that allows the cavity frequency to be independent of cavity length. PA1 a rotating-wave accelerator using a relatively unknown rotating (or circularly polarized) type of microwave field which has constant, but rotating fields, to eliminate the need for bunched beams and short cavities while allowing the use of a spiraling moving beam. PA1 an improved system and method for accelerating charged particle beams using transverse-magnetic (TM.sub.110) circularly polarized (rotating-wave) electromagnetic fields. PA1 an improved system and method for producing a continuous stream of monochromatic high-energy charged particles forming a helical beam having axial and rotational motion of a beam spot, such a spot rotating temporally about the device axis with a frequency equal to the radiation frequency .omega., with the individual electrons rotating at the cyclotron frequency .OMEGA..sub.c. PA1 an improved system and method for providing acceleration of a charged particle beam employing a transverse-magnetic (TM.sub.101) rotating-wave field with a relatively short microwave cavity whose length, being frequency independent, can be arbitrarily selected so as to maximize beam acceleration. PA1 an improved system and method for providing a transverse-magnetic (TM.sub.110) rotating-wave cavity with a suitable length which allows the construction of a properly up-tapered non-uniform axial magnetic field around it that yields substantial beam acceleration. PA1 an improved system and method for providing for maximum acceleration of a charged particle beam, by setting the relativistic cyclotron frequency of the electrons throughout the beam path equal to the frequency of operation of the cavity, i.e., .OMEGA..sub.c =.omega. (this condition can be called "gyroresonance"). PA1 an improved system and method for providing a charged particle extractor means for converting a rotating and axially translating helix into a pure axially translating beam which can subsequently be directed towards a target by means such as magnetic mirroring techniques. PA1 an improved system and method for providing permanent magnet means to achieve the properly shaped magnetic field profile for beam acceleration and extraction. PA1 an improved system and method for providing a compact charged particle accelerator which can be used for a large number of industrial, medical and defense applications. These applications include but are not limited to x-ray machines for medical radiotherapy, explosive detection, oil logging, structural inspection of airplanes, bridges, and other structures, electron beam machines for ionizing radiotherapy, electron beam welding, material hardening, food processing, sterilization of disposable medical products, and other applications.
A traveling wave cyclotron accelerator may, for example, use a cylindrical waveguide containing a circularly polarized transverse-electric traveling mode microwave wave as the means to produce beam acceleration. One limitation that such traveling wave cyclotron accelerators present is that, for a reasonable amount of input microwave power, the microwave electric field inside the waveguide is relatively weak. These fields are not strong enough to produce rapid acceleration of the particles--and thus require a long interaction to produce substantial acceleration of the particle beam. For instance, one example cyclotron accelerator using a 70 cm-long waveguide operating in a TE.sub.11 mode has been able to accelerate an electron beam up to 360 keV but requires 5 megawatts (that is 5 million watts) of microwave power. See Hirshfield, J. et al., Phys. Plasmas, 3, 1996, pp. 2163-2168. Although these devices can efficiently produce beam acceleration, they are typically large (at least in part because of the high microwave power required) and have only been able to produce low levels of energy gain. This is a big disadvantage for applications where compact and lightweight accelerating structures are required for the production of high-energy charged particles.
Cyclotron accelerators have been constructed using a microwave cavity employing a short cylindrical resonator holding a TE.sub.111 circularly polarized mode for particle acceleration. These cavity accelerators are much more compact than their traveling wave counterparts. However, one drawback of these cavities is that their dimensions (cavity radius and length) are both frequency dependent. That is to say, at a given frequency of operation, the cavity length becomes rather short if a reasonable cavity cross-section (radius) is to be obtained. It becomes very difficult to construct suitable magnetic coils around the short cavity to provide the required non-uniform up-tapered axial magnetic field profile to maintain cyclotron resonance throughout the beam path. Consequently, cavity cyclotron accelerators are forced to use a constant magnetic field whose amplitude is selected to maximize beam acceleration. Because of these reasons, the condition given by Eq. 1 above cannot be satisfied throughout the beam path and only low energy gains can generally be achieved with this type of accelerators. For example, a cavity cyclotron accelerator experiment designed to operate at a frequency of 2.82 GHz, employing a cylindrical cavity with a radius and length of 3.8 cm and 9.3 cm, respectively, yielded electron beam acceleration up to 500 keV using a uniform magnetic field of 1.4 kG. See Mc Dermott, D. B., et al., J. Appl. Phys. 58, 1985, pp. 4501-4508.